I use 50mm f/1.8 lens(with Canon T3i) for portraiture. It's quite easy when only one subject is involved :Open the aperture wide, focus on the eyes(most of the times) and you get a nice portrait.

How do I do it when more than one person is involved ? Suppose if there are two persons facing the camera,where do I focus ? I want a nice blurred background; If you focus on one person, the other person becomes a little blurred(which is not desirable). How do I get sharp image of both of them in the portrait with a reasonable blurred background.

Also I noticed that, when I am in Auto mode, the camera automatically selects two focus points , if two human faces are in the scene. Can I select more than one focus point in Av,Tv or Program mode ?

Related/possible duplicate: How can I get everything in focus? Also, you're asking two questions here: one about DoF and one about how to select multiple focus points - they'd be better off as two questions.
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Philip KendallFeb 21 '13 at 18:53

You could also adjust the tilt element of a lens with that capability to move the focal plane until it intersects both faces. ;-). In practice, neither easy nor cheap.
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Michael ClarkFeb 21 '13 at 22:38

If you want multiple points that aren't all in the same plane to be in focus, you'll probably need to use a somewhat smaller aperture.

Most lenses are normally wide open so that you get plenty of light to focus with, and so that you can more easily see exactly where the focus point is. To get a better idea of what will be acceptably sharp when you take the picture, you'll want to stop the lens down to your selected aperture. Every SLR that I've ever come across, digital or otherwise, has a button that you can press to close the aperture down to what you've selected. You'll know when you've hit it because the image in the viewfinder will get darker. You'll then be able to see whether all your subjects are in focus.

Your best bet is probably to use a Depth Of Field calculator to make sure you have sufficient depth of field to get both subjects sharp. After that, I'd focus on one person in One Shot mode and then adjust the focus manually as necessary to center up the focal plain.

If it is possible, you could try to turn a bit (or the people), so the plane that is in focus 'lays' on both persons. Although it will be difficult, you maintain your shallow depth of field and your shorter shutter time.

When that method is not possible or cumbersome, you should have to switch to another method. You could just focus on one and let the other be (sleightly) out of focus. When this does not result in the ideal picture for you, you could manually adjust the focus between both persons and/or stop down your aperture. You will certainly lose some light and out of focus blur, but when both persons have to be in focus, this is almost impossible to avoid. A depth of field calculator really helps choosing your aperture

When you get trouble with your exposure (inside buildings for example), you could consider using a flash or higher iso.

Aperture priority doesn't automatically mean "widest aperture". It means "use the aperture I tell you to". If you need more in focus, you have no choice but to set a narrower aperture for more depth of field.

If you want two people on focus but still want I blue the background, you have two choices, plus two alternatives:

Put the people next to each other, so that they are in the same plane of focus.

Use a narrower aperture for more DoF, but make sure the background is far enough away that it is still blurred.